Generally mineral wool products are manufactured in that a mineral material is melted in a melting apparatus from which the melted material is continuously supplied to a fribrillation apparatus. In said fibrillation apparatus the melted material is transformed, more or less completely, to mineralic fibres which, by means of an air or gas stream, are brought from the fibrillation apparatus to a receiver in the form of a perforated conveyor on which the fibres lay down in the form of a continuous mat of mineral fibres, whereas the air or gas stream moves through the conveyor and to an exhaust fan and is from there transported e.g. to a purification apparatus.
The raw materials for the mineral wool consist of stone or slag, and cupola furnaces usually are used as melting apparatus for such materials. In this case the fibrillation is often made by means of rapidly rotating steel cylinders, called spinning wheels, which are water-cooled from inside the cylinders, and which are arranged to successively receive the melted material. The melted material is thereby moved to one of said spinning wheels from which it is thrown to the next spinning wheel, etc. This type of arrangement is called the cascade spinning method or the JM-process (JM for Johns-Manville). There are generally 2-4 spinning wheels in the cascade spinning apparatus. Only rarely more than 4 spinning wheels are provided in cascade relationship.
Parallelly arranged fibrillation apparatus can be used in order to obtain a higher capacity. Generally, two identical or mirror symmetrical fibrillation apparatus are thereby used, which apparatus are placed side by side and between which the melted material is distributed.
In the manufacture of glass wool there is used more or less convention glass raw materials which are usually melted in a tank. In the most usual fibrillation method for glass wool the melted material is allowed to flow into hollow centrifuges which are mounted for rotation about vertical shafts. There are holes in the jacket of the centrifuges and the melted glass is by the centrifugal force pressed out through said holes thereby forming thin mineral strings, called primary fibres. The actual fibrillation is thereafter accomplished by means of strong axially directed gas or air jets which flow past the jackets of the centrifuges.
Also in the fibrillation of stone or slag mineral wool air flows are supplied round the spinning wheels. There are several reasons for supplying such air flows, for instance for bringing the fibres away from the actual fibrillation apparatus and to the collection means, whereby, as mentioned above, the fibres deposit on the collection means, whereas the gas or air streams pass through the collection means.
In commercial processes for the manufacture of mineral wool products the fibres, just formed, are more or less well suspended in such an air or gas stream, which brings the fibres from the fibrillation apparatus to the receiver means. If a binder is to be added to the mineral wool fibres, and binder is almost always added, the binder is mostly supplied in a finely dispersed form to said fibre suspension. Other substances may be supplied instead of a binder, or in combination with a binder, for instance a wetting agent or a dust binding substance. Often the binder is supplied very close the fibrillation apparatus. The reason for supplying the binder to the fibre suspension, and not to the mat already formed on the receiving conveyor is that is is difficult to have the binder penetrate into a mineral wool mat since such mats are tight and the fibres have a fine structure as compared with the size of the binder drops.
There are basically two different principles for the function of the receiver means. According to one principle, the originally used principle, a final mat is built up on the conveyor of the receiver means, and said final mat is then moved on where it is hardened, cooled, divided and cross cut etc. Technically this means that the surface weight, for instance defined in g/m.sup.2, of the the mineral wool when originally laid down on the perforated conveyor is the same as the intended surface weight of the final product already. In many cases the second principle gives a better result, and according to this principle a relatively thin mat, a so called primary mat, is first collected, and said primary path is then, by a folding process, in one or more steps, operated so as to build up a final mineral wool mat, the surface weight of which corresponds to intended surface weight of the final product. The surface weight of the primary mineral wool mat may, in many cases, be much less than that of the final mineral wool mat, for instance 1/10 thereof or less.
The stream of air or gas which brings the mineral fibres from the fibrillation apparatus to the receiver means is more or less turbulent. Tests have shown that the fibres are not completely mixed in the air or gas flow, in particular because the distance between the fibrillation means the the receiver means is not vary long. This means that fibres which are introduced in the said air or gas stream in a certain position tend to deposit at a special point or within a special area of the receiver conveyor, whereas fibres which are introduced in the air or gas flow in another position mainly deposit within another area of the receiver conveyor. This observation has been utilized as a basis for the present invention.
The binder of a mineral wool product is often used firstly to prepare the mineral wool product for its final field of use. For instance, after a mineral wool plate has been inserted between studs and behind final layers of a wall the binder has practically no function at all. In other cases the binder primarily is often supplied for transport technical and mounting technical reasons and only secondarily to fulfil demands of the product in its mounted condition. Anyhow a completely constant and homogenously distributed binder does not always give an optimum product. In cases when there is no need for the same content of binder all over the mineral wool product the inner portion of the product has the least need for binder.
It has been suggested that a binder which is homogeniously distributed in a mineral wool product manufactured according to normal methods should be be completed by an extra supply of binder which is subsequently added on the surfaces of the ready product. Said additional binder has to be dried or hardened before the product is ready, and therefore this is a two stage method. An advantage in such a method is that the main binder can be supplied in smaller amounts than is otherwise necessary. The gain thereof is, however, consumed by the costs for executing the said two stage method.